1. Field:
Embodiments of the disclosure relate to the formation of a composite part and, more particularly, to apparatus and methods for forming hat stiffened composite parts using stretchable tooling cauls.
2. Background:
Composite structures, such as those utilized in the aircraft and other industries, are commonly formed by curing a polymeric composite material while the material is covered, at least partially, and supported by forming tools. One common composite structure employed in aerospace and other applications is a hat stiffened composite part. To construct a hat stiffened composite part, the fabrication process begins by placing composite material on a base tool, placing a tooling mandrel on the composite material supported by the base tool, placing additional composite material over one tooling mandrel, and covering at least part of the composite material with a forming tool. The forming tool usually covers at least the composite material that overlays the tooling mandrel and helps define the hat shape. The composite material may then be subjected to a curing process, such as debulking and heating, to further adhere and bond the composite layers in order to produce an integral composite structure.
The increased accuracy required for critical dimensions of parts has created a need for tools that can form composite parts with high degrees of accuracy, at least for the critical dimensions. With respect to hat stiffened composite parts, for example, the radius defined at the intersection between the hat stiffener legs and the corresponding planar structure must typically be maintained to within a very tight tolerance. Conventional rigid forming tools have provided limited capabilities to produce composite parts within the required accuracy. Namely, during the curing process, the tooling mandrel expands while the less thermally expansive composite forming tool, which covers at least a portion of the composite material, expands to a lesser degree, resulting in high and low pressure areas within the composite material that cause part porosity or unwanted composite material movement. As such, composite material may be scrapped in failed attempts to produce parts having the critical dimensions using an almost non-expansive forming tool. In addition, the difference in expansion between the tooling mandrel and the less thermally expansive forming tool may also impair the bonding process of the composite material so that the resulting part exhibits flaws, such as disbonds.
More thermally expansive forming tools have been developed that are capable of more expansion than less thermally expansive tools. However, both nearly non-expansive and more thermally expansive forming tools fail to consistently produce parts that meet the critical dimensional requirements. For instance, a hat stiffened composite part generally requires a specific corner radius at the intersection of the hat stiffener legs and the corresponding planar structure. This is often a critical dimension for a hat stiffened composite part. Generally, a thermally expansive tooling mandrel may be used to form the inside of the hat structure, while a less thermally expansive forming tool may be used to form the outside of the hat structure. Often, the thermally expansive tooling mandrel expands during the curing process, while the less thermally expansive forming tool does not expand nearly as much. The problem with this difference in expansion is that the corner radius of the finished part is out of tolerance.
It would therefore be advantageous to provide apparatus and methods for forming a composite part, such as a hat stiffened composite, with increased accuracy of formation and quality. In addition, it would be advantageous to provide apparatus and methods to decrease the amount of composite material scrapped because of poor part formation techniques and tools.